The California Environmental Protection Agency's Air Resources Board (ARB) today identified particulate emissions from diesel-fueled engines as a toxic air contaminant. Approval by the 11-member board ends a near-decade long scientific investigation into the health effects of exposure to the fine particles and other pollutants in diesel exhaust.

ARB Chairman John Dunlap said, "This closes the identification phase of the examination on the health effects of diesel emissions and allows the ARB to reinvigorate efforts to find additional ways to protect public health. "As part of that process, we will review all existing regulations and emerging technologies to determine if there are further opportunities to reduce emissions from diesel engines," he added.

The ARB concluded its hearing on the matter, which began at last month's meeting. The Board heard more than six hours of testimony and discussion, including presentations by ARB staff, Office of Environmental Health Hazard Assessment and the independent Scientific Review Panel (SRP), which monitored the report's development. A decision was postponed last month after more than 60 state legislators asked the ARB to defer a decision until two legislative committee hearings were held on the issue.

During the past month, a proposal was crafted by environmentalists and industry stakeholders, with the aid of ARB staff, which refined the initial proposal to focus on diesel particulate. The tightened proposal will allow for more targeted control efforts and eliminates harmless exhaust components, such as water vapor, without lessening the ARB's ability to control diesel emissions.

"That change gives industry a target, the fine particles in diesel exhaust, to pinpoint their control efforts without compromising California's ability to appropriately regulate any other toxic compounds found in diesel exhaust," Dunlap added.

Because of its significant contributions to photochemical smog and fine particle pollution, both diesel fuel and engine technology are already regulated. As a result, today's diesel engines and fuels emit significantly less pollution than comparable technology available 20 years ago.

As a result of this identification, state law requires a review of current and expected regulations to determine potential control strategies. An advisory group is being formed comprised of state and local air quality officials, environmentalists, business and industry representatives. They are expected to begin work by next spring.